Abstract

Boulder barricades are elongate rows of boulders that flank the coastline, separated from the shore by a low gradient intertidal zone. Bouldery flats are low gradient areas with randomly-deposited boulders. Both are the result of ice transport and therefore are common features of certain Arctic or sub-Arctic regions. They are formed by the grounding of boulder-laden ice rafts in nearshore zones during spring ice break-up. In tidal areas, such as a Labrador fjord, intertidal ice freezes downward, with continued freezing at each high tide. Boulders are encased in the ice at the bottom. Observations on a broad intertidal flat indicate that an increased number of boulders are lifted from the upper intertidal zone. Apparently, the less frequent lifting of the ice during spring tides is more effective at encasing boulders than the diurnal lifting in the lower intertidal zone. High melting rates occur from the ice surface in early spring, so the combined effect of freeze down and surface melt result in the transportation of boulders up through the ice. In spring, the intertidal ice breaks up before the floating ice due to river action, numerous tidal cracks and the decreased albedo of the mud-laden nearshore ice. Shore leads up to one km wide serve as thoroughfares for wind- and tide-transported ice rafts. The boulders encased in the ice are often deposited as random bouldery flats. However, some areas have a distinct break-in-slope near the low water line. As the ice thickness is comparable to the mean tide range, the highest probability for ice rafts to ground is at this slope break. Accumulation of boulders at this position over successive seasons results in the formation of boulder barricades. The requisite conditions for the formation of boulder barricades derived from this field setting are a) a rocky coastal setting, to serve as a source for boulders; b) sufficient winter ice and water level fluctuations to entrain boulders in ice rafts; and c) a distinct break-in-slope in the nearshore zone. Without the third condition, boulders were deposited randomly as bouldery tidal flats.